Well, clearly this building isn't actually going to be 1,776' tall.
Main Lobby is at Elv. 311'2
North Entrance is at 307'10"
Beacon is at 2087'2, 1776' above main lobby floor, but 1779'4 above north entrance.
PLUS, if you look at the top of the diagram, there's an 8' section above the beacon, making it 1787'4" tall.

1787 is actually a much better number, when you think about it. 1776 was when we declared war on a country that's now an ally. How the hell is that really relevant to this building? 1787, on the other hand, was the year the US Constitution was written and adopted. Much more relevant.

what datum is the project working off of for lobby to be at elevation +311'-2" and spire to be at +2087'-2"? clearly not the manhattan datum...

update/edit:
a few minutes on google and i've answered my own question...http://www.pobonline.com/Articles/Co...100000f932a8c0
"Most of the work in New York City, however, is tied to a specific borough datum. In Manhattan for example, the Manhattan vertical datum is 2.75' above mean sea level at Sandy Hook. Transit Authority vertical datum is 100.1' above the Manhattan vertical datum. Similarly, the Port Authority vertical datum is 297.347' above NGVD 29. The Transit Authority and Port Authority, with tunnels well below sea level, use these datums so engineers and surveyors can work in positive numbers."